A recent development in printing technology is the process of profiling hardcopy printing devices. In the profiling of hardcopy printing devices, a broad selection of color patches is printed that samples the entire printing gamut. The colors of these patches are measured. The measured color is then used to either modify the colors of a proof such that the proof matches more closely with the printed product, or to modify the colors produced on press of the printed product such that the product matches more closely with the target colors.
Currently in order to profile a gravure printing press, a special and costly test run is required to produce the test target.
A factor that can effect a printing press profile is the coating on the paper stock which can have a large influence on ink color printed. Other factors such as paper color and ink formulation also have an effect on the color printed on the paper. Further, the profile of a given press gradually changes over time, and may require constant and frequent updating. Updating a profile is a costly and time consuming process.
In a typical gravure printing press, concentrated ink and solvent are mixed in a sump. The concentrated ink includes pigments, varnish, wax, surfactant and the like. The solvent is used to thin the concentrated ink to a proper consistency. Factors that effect color on a gravure press include the variety of sizes of the gravure cells, the pigment concentration of the ink, and the viscosity of the ink/solvent mixture. Among the effect associated with the viscosity of the ink/mixture is that less viscous mixture leaves the gravure cells and transfers to the paper much more readily than mixture that is more viscous. Specifically, variations in viscocity will have an effect on the highlight cells. An increase in viscocity will reduce the amount of ink released from a highlight cell far more than the same increase in visocity will effect the ink released from a shadow.
Further, solvent is generally volatile, and therefore, has to be replenished frequently. As a result, gravure presses are equipped with a control mechanism to maintain a constant viscosity of the ink and solvent mixture through the addition of solvent.
When a float valve coupled to the sump senses that the volume of ink has dropped below a predetermined level, the printing press will perform a dump process. In the dump process, a controlled amount of ink is added to the sump through the float valve.
Viscosity control mechanisms generally employ some variation of a Zahn cup or the like as a viscometer. The viscosity of the ink is determined by measuring the time required for a measured amount of the ink to pass through a small precision orifice. For example, the pressure of the ink activates mechanical switches used for timing the egress of the ink. Alternatively, an emitter-detector pair and associated electronics perform the same function. Other applications use a hydrometer to measure a specific gravity of the ink that is directly related to the viscosity of the ink.
In addition to viscosity, ink temperature can also have an effect on gravure press color. As ink temperature rises, ink generally becomes less viscous. As the ink and solvent mixture becomes less viscous, the highlights and shadows of the print work will be affected. Since high pressure at the nip and the high speed of a gravure print cylinder create heat, a temperature control mechanism is typically used to control the temperature of the ink in the range of about 70° F. to about 80° F. The control mechanism generally consists of a controlled flow of chilled water through pipes that weave back and forth through the sump.
More specifically, several different print parameters are controlled in a gravure press. One of these parameters is the optical density of the shadows of the print work. Another such print parameter is the optical density of the highlights. The ink concentration and the viscosity of the mixture will generally affect these parameters. Particularly, the concentration affects the shadow of the print work. On the other hand, the viscocity affects mainly the color of the highlights. However, prior ink control systems incorporate ink density control but do not deal adequately with viscosity control. This does not allow for adjusting ink to the proper formulation in order to achieve proper highlights and shadows.
In some gravure printing presses, there exists the ability to combine three fluids in the ink fountain: a concentrated ink, a solvent, and an extender. The extender is essentially a concentrated ink without the pigment. By mixing the concentrated ink and the extender on press, it is possible to meet the particular needs of a variety of customers without specially pre-mixed formulations.
The addition of the three fluids on such a gravure press is typically controlled at two levels. In the sump, a viscometer controls the addition of solvent to make up for the rapid evaporation of the solvent. A level indicator controls the addition of ink. If the ink level drops below a certain point, fixed volumes of concentrated ink, extender and solvent are added, respectively. In general, a press operator sets the relative volumes of these fluids at the beginning of a printing run. However, while the blending of the mixture is automatic, a pressman is required to monitor the print work, to modify and to set the mixture when necessary.